1. Field of the Invention
The present invention relates in general to a plasma generating device and a plasma processing method.
2. Description of the Prior Art
Self-sustaining electric discharge takes place through a gas at air pressure when a high electric field is applied thereto, e.g. in the form of DC, AC or impulses. This kind of discharge can be sorted into corona discharge, glow discharge and arc discharge. If an electric field is applied uniformly through the gas, dielectric breakdown takes place uniformly to cause glow or arc discharge. If the electric field is partially strong, dielectric breakdown locally takes place to cause corona discharge. The corona discharge can be changed into uniform dielectric breakdown, i.e. arc discharge. The transition from corona discharge to arc discharge usually takes place directly rather than via glow discharge. This is because a very large number of ions collide with an electrode at a high pressure (atmospheric pressure), as compared with the case at a low pressure, and quickly elevate the temperature of the electrode to cause thermoelectronic emission, which characterizes arc discharge. It is known that glow discharge can be utilized when current is limited below 2A. Glow discharge, however, can not be easily controlled so that arc discharge is mainly utilized in actual applications such as welding, cutting.
Arc discharge is utilized to melt a work piece by virtue of high temperatures of the electrode and its positive column (gas temperature). The temperature of the work piece therefore necessarily rises to a very high temperature, e.g. 2000 to 6000 K.
An attempt has been carried out to stabilize glow discharge at air pressure to make it possible to carry out machining at room temperature (S. Kanazawa et al. J. Phys. D: Applied Physics 21(1988), pp. 838-840). In order to cause stabilized glow discharge at air pressure, the following conditions must be satisfied. 1. The discharging space is filled with He. 2. An insulator is interposed between electrodes (across the discharge path). 3. At least one of the electrodes is provided with its end in the form of a needle or a brush. 4. The frequency of the applied electric field is no lower than 3 KHz. The second condition is necessary for avoiding transition into arc discharge. The fourth condition is necessary for enabling current passing through the insulator. The third condition is necessary for making it easy to initiate discharge by forming a non-uniform electric field. Attempt has been made to give etching treatment to surfaces of organic substances such as polyimide or inorganic substances such as silicon.
These known methods, however, must be carried out by evacuating the reaction space in advance followed by introducing a reactive gas such as He. The existing methods treat the entirety of a substrate and can not selectively process small areas of the substrate.